The present invention relates to a method and an installation for the treatment of molten metal, particularly of steel in a ladle.
At present, a series of methods exists for the treatment of molten metal, particularly of steel in the ladle, according to which a bell or a tube is plunged into the molten metal contained in a ladle. Such methods of treatment include, inter alia, those known as CAS, CAS-OB, HALT, etc.
In this type of method, the molten metal contained in the ladle is subjected to different treatments in a confined zone, defined by the bell plunging into the molten metal. A bubbling gas is injected under the bell into the molten metal in order to homogenise it during the treatment. Turbulence then occurs at the surface of the molten metal which leads to an increased local wear of the bell over its lower edge.
The objective of the present invention is to propose a method and a device making it possible to minimise the local wear of the bell during the treatment of molten metal in a ladle.
This objective is attained by a method aiming to minimise the local wear of a bell during a treatment of a molten metal in a ladle, characterised in that the bell is rotated about an axis during the treatment of the molten metal.
The fact that the bell is given a rotational movement enables the local wear of the bell to be minimised. In effect, since the bell rotates during the treatment of the molten metal, increased wear at a given place due to turbulence in the molten metal is no longer a source of concern. The bell is in fact worn uniformly over the whole of its perimeter.
Since these bells are costly and since the replacement of worn bells takes a certain amount of time, the present method also enables the running costs of the installation to be reduced.
Such a method is particularly useful when implementing the method for the treatment of molten steel in a ladle described in the European patent EP 0 110 809. In such a method, by which the steel contained in the ladle is heated by the aluminothermic process and by which a certain number of alloying elements are added to the steel, the bell is asymmetrically stressed:
on a xe2x80x9chotxe2x80x9d side, the bell, or more precisely the refractory lining of the lower edge of the bell, is attacked by thermal shocks and by chemical corrosion produced by splashes of metal and slag. The wear is caused mainly by spalling of the refractory lining.
on the xe2x80x9ccoldxe2x80x9d side, and possibly in intermittent usage when the refractory lining is cooled, the bell is xe2x80x9cfattenedxe2x80x9d by solidification of splashes of metal and/or of slag.
These phenomena of local wear and xe2x80x9cfatteningxe2x80x9d considerably reduce the working life of the bell and thus increase the costs of production by the method involving addition and heating under the bell. The proposed method prolongs the useful life of a refractory bell by minimising the local wear, by reducing the local xe2x80x9cfatteningxe2x80x9d and even by compensating for local wear by a lining produced in situ.
According to a first advantageous mode of execution, the rotational speed of the bell lies between 0.5 and 2 revolutions per minute during the treatment of the molten metal. The rotational speed of the bell may be adapted as a function of the diameter of the bell, as a function of the treatment applied to the molten metal, and/or as a function of the composition and viscosity of the slag covering the molten metal in the ladle. Of course, the ladle may continue to rotate about its axis even when it is withdrawn from the bath after the treatment of the molten metal.
According to another preferred mode of execution, the bell rotates about a vertical axis, roughly perpendicular to the surface of the molten metal or the molten steel.
According to another aspect of the present invention, an installation is also proposed for the implementation of the method, the said installation incorporating a driving device for driving the bell in a rotational movement during the treatment of the molten metal.